Disposal of chemical wastes has been a problem because of space limitations, contamination of natural resources and considerable financial burden to industries and communities generating such wastes. A large fraction of wastes which must be disposed of are aqueous solutions, including sludges and slurries, which pose problems of weight and volume, thus being expensive to transport and difficult to contain over long periods of time.
Common disposal methods of wastes, such as sludges and other dilute aqueous wastes, include landfilling, deep well injection and incineration. Neither landfilling nor deep well injection eliminates these materials. Incineration is often limited to relatively concentrated aqueous wastes and can release harmful, partially oxidized, reaction products.
Oxidation of wastes by reaction of organic materials and oxygen in the presence of water at high temperature and pressure is an alternative method of disposal. One such method is the Zimmerman wet air oxidation process. See, for example, Wilhelmi et al., "Wet Air Oxidation-An Alternative to Incinerational," Chem. Eng. Prog., 75: 46-52 (1979). In the Zimmerman process, a two-phase feed mixture, including waste material, water and air, is pressurized by an air compressor and a high pressure pump, and then heated in a feed/product heat exchanger. The pressurized and heated feed mixture is then passed through a reactor where the temperature is sufficient to partially oxidize the waste material by reaction with oxygen. An effluent mixture is removed from the reactor and directed through the heat exchanger for cooldown and for transfer of heat from the effluent to the feed mixture. The heat exchanger is either a tube-and-shell or a tube-in-tube regenerative-type heat exchanger.
Another method for oxidation of organic material includes reaction with oxygen at supercritical conditions for water. See, for example, Thomason et al., "Supercritical Water Destruction of Aqueous Wastes," Hazardous-Waste 1: 453-467 (1984). In this process, a dilute aqueous waste is heated in a counter-current heat exchanger and mixed with pressurized air to form a feed mixture. The feed mixture is then passed through an oxidizer, wherein the organic material is oxidized at a temperature and pressure in excess of the supercritical point for water. Inorganic solids, that may have been present in the feed, and inorganic salts, that may have formed as a solid precipitate in the oxidizer, are separated out at elevated temperature. One process teaches removal of such inorganic solids in a cyclone which receives the reactor effluent. See, for example, U.S. Pat. No. 4,338,199. However, such embodiments have been known to become clogged as a result of buildup of solids on reactor and cyclone walls. See, for example, MODAR, Inc. "Supercritical Water Oxidation: Gravity Independent Solids Separation." Final Report to NASA for the Period May 1, 1987 to Apr. 30, 1988 under Contract NAS2-12176; Stone & Webster Engineering Corporation. "Assessment and Development of an Industrial Wet Oxidation System for Burning Waste and Low-Grade Fuels." Report No. DOE/ID/12711-1 to the department of Energy for Work Performed under Contract No. DE-FC07-88ID12711. Another process uses a large vessel reactor which provides a somewhat quiescent environment in which solid particulate fall to the bottom of the vessel (U.S. Pat. No. 4,822,497). The bottom of the vessel is maintained below the supercritical temperature so that some water will condense and form a pool of concentrated brine by dissolving some of the settled salts. The hot brine solution is removed through a valve at the bottom of the vessel reactor. There are several problems with this arrangement: (1) the temperature gradient from the top to the bottom of the vessel reactor is a source of heat loss and can significantly reduce the potential for recovering useful energy; (2) the hot brine is very corrosive and depressurization of such a brine through a valve is problematic; (3) the flow pattern in the vessel reactor does not ensure complete oxidation of organic matter so that a second stage reactor is usually required to further oxidize the fluid effluent from the vessel reactor; and (4) the brine may likewise contain unoxidized or partially oxidized products and may require special disposal if it is deemed hazardous.
Following oxidation and removal of inorganic solids, an effluent mixture is thus formed which is passed through a feed/product heat exchanger for cooling of the effluent mixture and for transfer of heat to the feed mixture. The effluent mixture leaving the feed/product heat exchanger can pass through high and low pressure liquid/vapor separators. Liquid and gas streams leaving the separators can be depressurized at pressure letdown valves or through an expander turbine.
A problem common to the above prior art processes of oxidation in the presence of water is the presence of inorganic salts, which can constitute a substantial portion of solids generated during oxidation of wastes. Many of these salts exhibit inverse solubility, being less soluble at higher temperature. An example of such an inorganic salt is calcium sulfate, which is a common component of "hard water." A solution of water and about 0.1 weight percent calcium sulfate, for example, will form solid .alpha.-anhydrite at temperatures above 85.degree. C., which collects as scale on heat transfer surfaces of boilers because the heat transfer surfaces are hotter than the proximate feed stream temperature. Another inorganic salt which exhibits reverse solubility is sodium chloride (NaCl). At a pressure of about 200 atmospheres, for example, the solubility of sodium chloride decreases sharply from about 50 weight percent, at a temperature below 400.degree. C., to about 0.01 weight percent above 400.degree. C. Examples of common inorganic salts in which solubility with water decreases from about 20-30 weight percent at 200 .degree. C., to very small solubilities at temperatures above 300.degree. C. include sodium phosphate (Na.sub.3 PO.sub.4), sodium carbonate (Na.sub.2 CO.sub.3), sodium sulfate (Na.sub.2 SO.sub.4) and potassium sulfate (K.sub.2 SO.sub.4).
Reduction of pressure at high temperatures in the reactor effluent by the above prior art processes has often been impractical because inorganic salts can obstruct flow through most depressurizing means, such as letdown valves and turbines, and consequently must first be separated from the effluent stream. Further, the presence of inorganic salts at high temperatures can also form a brine which is very corrosive, which, in turn, can reduce the useful life of apparatus exposed to these conditions. Also, the hot brine removed from a reactor or solid/liquid separating apparatus can be classified as a hazardous material in the event hazardous by-products are contained therein.
In addition, heating of some organic materials, when dissolved in water, will form a char. See, for example, U.S. Pat. No. 4,113,446. Char is often formed by depletion of hydrogen relative to the presence of carbon in organic material, and can accumulate on heat transfer surfaces of heat exchangers used for oxidation of wastes.
Other materials which are present in some wastes and which can settle from a reaction stream include silica, alumina, and oxides and carbonates of transition metals, heavy metals and rare earth metals. Metal oxides can form and precipitate from solutions when present-as dissolved salts in feed streams, including, for example, organic ligands, such as ethylene diamine tetracetic acid, or inorganic complexing agents such as ammonia. Insoluble carbonates can form by combination of metals with carbon dioxide in the reactors or in cool-down heat exchangers. These materials are usually insoluble in both liquid water and water above the supercritical point.
Inorganic salts, char and metal-containing solids can accumulate on the walls of apparatus, thus limiting the operating capacity and useful life of such apparatus. Removal of accumulated deposits and scale within reaction systems can necessitate dismantling and cleaning of apparatus in order to remove hard scale caused by a prolonged presence of insoluble salts. In some cases, inorganic acids have been used to dissolve and remove scale. Consequent downtime can be expensive, time consuming, and can cause associated problems typical of shutdown and startup of continuous chemical processing. Further, the materials which must be used to remove deposits and scale from apparatus are themselves often highly corrosive, thereby necessitating frequent repair and replacement of such apparatus.
Thus, a need exists for an improved method and apparatus for oxidation of organic materials in the presence of inorganic materials in an aqueous stream which overcomes or minimizes the aforementioned problems.